1. Field of the Invention
The present invention relates to a display apparatus in which a current light-emitting element is driven with a thin-film transistor (current-drive thin-film transistor display apparatus).
2. Description of the Related Art
A number of thin-film transistor display apparatuses of various types for realizing lightness in weight, smallness in size, high image qualities and high resolution have been used. Thin-film transistor display apparatuses hitherto developed, as represented by thin-film transistor liquid crystal displays, are mainly for transmission of signal voltages or transfer of minute charges. However, it is anticipated that an element capable of current driving and having a memory function will become indispensable to self-light-emitting type panels, such as EL (electroluminescence) displays, heat-developing panels and the like which are expected to be developed in future.
FIGS. 10(a) and 10(b) are an equivalent circuit diagram and a potential relationship diagram, respectively, of a current-drive thin film transistor display apparatus, in which an organic fluorescent material is used as a light emitting material.
In FIG. 10(a), symbol 121 represents a scanning line; symbol 122, a signal line, symbol 123, a common current supply line; symbol 131, a switching thin-film transistor; symbol 132, a current thin-film transistor; symbol 151, a holding capacitor; symbol 152, a pixel electrode; symbol 164, an organic fluorescent material; and symbol 165, an opposed electrode. In FIG. 10(b), line 421 represents a scanning potential; line 422, a signal potential; line 423, a common potential; line 451, a held potential; line 452, a pixel potential; and line 465, a counter potential.
The switching thin-film transistor 131 is a transistor for controlling conduction between the signal line 122 and holding capacitor 151 by a potential on the scanning line 122. That is, signal potential 422 is transmitted to held potential 451 by scanning potential 421. With respect to a displaying pixel, signal potential 422 becomes high and held potential 451 becomes high. With respect to a non-displaying pixel, signal potential 422 becomes low and held potential 451 becomes low.
On the other hand, the current thin-film transistor 132 is a transistor for controlling conduction between the common current supply line 123 and the pixel electrode 152 by the potential on the holding capacitor 151. That is, common potential 423 is transmitted to pixel potential 452 by held potential 451. With respect to a displaying pixel, conduction is effected between the common current supply line 123 and the pixel electrode 152. With respect to a non-displaying pixel, the common current supply line 123 and the pixel electrode 152 are shut off from each other.
Consequently, a current is caused to flow between the pixel electrode 152 and the opposed electrode 165 with respect to a displaying pixel, thereby causing the organic fluorescent material 164 to emit light. With respect to a non-displaying pixel, no current flows and emission of light is not caused.
Thus, the current-drive thin film transistor display apparatus has the switching thin film transistor 131 and the current thin-film transistor 132, each of which is a field effect type transistor manufactured by an ordinary semiconductor manufacturing process. As such, for the two thin-film transistors for conventional current-drive thin film transistor display apparatuses, thin-film transistors of the same structure have been used because the manufacturing cost can be reduced if the two transistors are made in accordance with the same specifications.
Actually, even if the structures of the two thin-film transistors are the same, the current-drive thin-film transistor display apparatus has no serious defect relating to it. However, if a high-quality product is aimed at, it is preferable to construct the above-described two thin-film transistors in such a manner that importance is attached to making the characteristics of the two transistors different from each other.
That is, with respect to switching thin-film transistor 131, a reduction in off-current is needed for the purpose of enabling the charge to be retained more reliably in the holding capacitor 151. In contrast, with respect to current thin-film transistor 132, an increase in on-current is needed for the purpose of increasing the luminance of emission of light from the organic fluorescent material 164.
However, no technical idea of positively making the characteristics of the above-described two thin-film transistors different from each other in a current-drive thin-film transistor display apparatus has been conceived.
The present invention has been achieved based on such a knowledge, and an object of the present invention is to provide a current-drive thin-film transistor display apparatus in which a reduction in the off-current of switching thin-film transistor 131 and an increase in the on-current of current thin-film transistor 132 are achieved simultaneously.
To achieve the above-described object, according to an aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, the switching thin-film transistor is designed in such a manner that importance is attached to reducing the off-current, and the current thin-film transistor is designed in such a manner that importance is attached to increasing the on-current.
According to another aspect of the invention, each of the switching thin-film transistor and the current thin-film transistor is constructed according to required performance of the transistor in such a manner that importance is attached to reducing the off-current or increasing the on-current, so that charge can be retained more reliably in the holding capacitor, and so that a sufficient current supply to the pixel electrode can be effected more reliably.
To achieve the above-described object, according to another aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a low concentration impurity region is formed between a channel region and a high concentration impurity region of the switching thin-film transistor, and a channel region and a high concentration impurity region of the current thin-film transistor are directly connected to each other.
That is, in the invention set forth in claim 2, the switching thin-film transistor is formed as a transistor of Lightly Doped Drain (LDD) structure while the current thin-film transistor is formed as a transistor of self-alignment structure.
According to another aspect of the invention, the switching thin-film transistor is designed so as to reduce the off-current while the current thin-film transistor is designed so as to increase the on-current, so that charge can be retained more reliably in the holding capacitor, and so that a sufficient current supply to the pixel electrode can be effected more reliably.
To achieve the above-described object, according to another aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a low concentration impurity region is formed between a channel region and a high concentration impurity region of each of the switching thin-film transistor and the current thin-film transistor, and the length of the low concentration impurity region of the switching thin-film transistor is increased relative to the length of the low concentration impurity region of the current thin-film transistor.
That is, each of the switching thin-film transistor and the current thin-film transistor is formed as a transistor of LDD structure, and the length of the low concentration impurity region (LDD length) of the switching thin-film transistor is increased relative to the LDD length of the current thin-film transistor.
To achieve the above-described object, according to another aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a region having an impurity concentration substantially the same as that of a channel region of the switching thin-film transistor is formed between the channel region and a high concentration impurity region of the switching thin-film transistor, and a channel region and a high concentration impurity region of the current thin-film transistor are directly connected to each other.
That is, the switching thin-film transistor is formed as a transistor of offset structure while the current thin-film transistor is formed as a transistor of self-alignment structure.
According to another aspect of the invention, the switching thin-film transistor is designed so as to reduce the off-current while the current thin-film transistor is designed so as to increase the on-current, so that charge can be retained more reliably in the holding capacitor, and so that a sufficient current supply to the pixel electrode can be effected more reliably.
To achieve the above-described object, according to another aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a region having a certain impurity concentration is formed between a channel region and a high concentration impurity region of each of the switching thin-film transistor and the current thin-film transistor, the impurity concentration of the region between the channel region and the high concentration impurity region being substantially the same as that of the channel region, and the length of the region having an impurity concentration substantially the same as that of the channel region of the switching thin-film transistor is increased relative to the length of the region having an impurity concentration substantially the same as that of the channel region of the current thin-film transistor.
That is, each of the switching thin-film transistor and the current thin-film transistor is formed as a transistor of offset structure, and the offset length of the switching thin-film transistor is increased relative to the offset length of the current thin-film transistor.
According to another aspect of the invention the holding capacitor is formed by using a gate insulating film between the scanning line and the channel region of the switching thin-film transistor or the current thin-film transistor.
According to another aspect of the invention, a thin gate insulating film is used for the holding capacitor to enable the holding capacitor to be formed as a small-area large-capacity capacitor.
In contrast, in the display apparatus according to another aspect of the invention, the holding capacitor is formed by using an interlevel insulating film between the scanning line and the signal line.
According to another aspect of the invention, an interlevel insulating film is used for the holding capacitor to improve the degree of freedom of designing.